[run_seq2seq.py] restore functionality: saving to test_generations.txt (#10428)

This PR restores the original functionality that for some reason was modified.

Fixes: https://github.com/huggingface/transformers/issues/10381

@sgugger

examples/seq2seq/run_seq2seq.py

@@ -629,7 +629,7 @@ def main():
                     test_results.predictions, skip_special_tokens=True, clean_up_tokenization_spaces=True
                 )
                 test_preds = [pred.strip() for pred in test_preds]
-                output_test_preds_file = os.path.join(training_args.output_dir, "test_preds_seq2seq.txt")
+                output_test_preds_file = os.path.join(training_args.output_dir, "test_generations.txt")
                 with open(output_test_preds_file, "w") as writer:
                     writer.write("\n".join(test_preds))